Books
From Trashwiki
For the literature about dumpster diving see here.
Books can often be found while walking through affluent Western cities. Just keep your eyes out for random boxes in the street. guaka has found books in Paris, Amsterdam, Cologne and several other cities.
Fun tip: if you don't want to keep the awesome books you found yourself, try giving them away to unsuspecting strangers.
There is even 'the world's biggest free book club', Book Crossing. jtredd has found books in Philadelphia, Pennsylvania and other places through Book Crossing.

